As compounds expectedly useful as an antihypertensive agent due to K ion channel opener activity, those represented by the following formula are known [U.S. Pat. No. 4,446,113; Br. J. Pharmac. (1986), 88 , 103-111; Br. J. Pharmac. (1986), 89, 395-405]: ##STR2## The purpose of the present invention is to develop new compounds having a K ion channel opener activity.